I. Field
The following description relates generally to wireless communications, and more particularly to mechanisms for inter-eNode B (eNB) handover.
II. Background
Wireless communication systems are widely deployed to provide various types of communication; for instance, voice and/or data may be provided via such wireless communication systems. A typical wireless communication system, or network, can provide multiple users access to one or more shared resources. For instance, these systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Generally, a wireless multiple-access communication system can support simultaneous communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink (DL)) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink (UL)) refers to the communication link from the terminals to the base stations. Such communication links can be established via a single-in-single-out, multiple-in-single-out or a multiple-in-multiple-out (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≦min{NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized. A MIMO system can support time division duplex (TDD) and frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows the estimation of the forward link channel from the reverse link channel. This enables the access point to extract transmit beamforming gain on the forward link when multiple antennas are available at the access point.
In cellular wireless systems, a service area is divided into a number of coverage zones generally referred to as cells. Each cell may be further subdivided into a number of sectors served by a number of base stations. While each sector is typically depicted as a distinct geographical area, sectors typically provide overlapping signal coverage to provide seamless communication as wireless terminals or user equipment (UE) transit from a cell to an adjacent cell. For example, when a mobile user passes between cells, there must be efficient communications handover or handoff between base stations to provide a seamless mobile internet experience to the user. Without an efficient mechanism to hand-off mobile users between cells, the user would experience service interruptions and delays, lost transmissions, or dropped calls.
A handoff, or handover (HO), is the process in which a UE (e.g., a wireless phone) is handed from one cell to the next in order to maintain a radio connection with the network. The variables that dictate a handover depend on the type of cellular system. For example, in CDMA systems interference requirements are the limiting factor for handover. In FDMA and TDMA systems such as the Global System for Mobile communications (GSM), the main limiting factor is the signal quality available to the UE.
One form of handover or handoff is when a UE call in progress is redirected from its current cell (e.g., the source cell) and channel to a new cell (e.g., the target cell) and channel. In terrestrial networks, the source and the target cells may be served from two different cell sites or from two different sectors of the same cell site. The former is called an inter-cell handover, where the latter refers to a handover within one sector or between different sectors of the same cell (e.g., an intra-cell handover). Generally, the purpose of inter-cell handover is to maintain the call as the subscriber is moving out of the area covered by the source cell and entering the area of the target cell.
As an example, during a call, one or more parameters of the signal in the channel in the source cell are monitored and assessed in order to decide when a handoff may be necessary (e.g., the DL and/or UL may be monitored). Typically, the handoff may be requested by the UE or by the base station of its source cell and, in some systems, by a base station of a neighboring cell. The phone and the base stations of the neighboring cells monitor each other others' signals and the best target candidates are selected among the neighboring cells.
For example, Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access Network (UTRAN) contains base stations (e.g., Node Bs), and Radio Network Controllers (RNC). The RNC provides control functionalities for one or more Node Bs and carries out Radio Resource Management (RRM), some of the mobility management functions, and is the point where encryption is done before user data is sent to and from the mobile. A Node B and an RNC can be the same device, although typical implementations have a separate RNC located in a central office serving multiple Node B's. The RNC and its corresponding Node Bs are called the Radio Network Subsystem (RNS). There can be more than one RNS present in an UTRAN. The UE requires a Radio Resource Control (RRC) connection to access the services of the UMTS network, which is a point to point bi directional connection between the RRC entities on the UE and UTRAN (e.g., the RRC is terminated in the UTRAN). Typically the UMTS handover mechanism (e.g., measurement, decision, and execution) is centrally controlled where the RNC is responsible for handover decisions, requiring handover signaling to the UE, and requiring complicated coordination via 3-way handshake (Measurement Report, Handover Command (HO Command), and HO Complete) among the network components.
One problem in connection with such a mechanism is that interoperability issues with deploying UTRAN equipment from different vendor have generally hindered mobile operators' attempts to deploy multi-vendor networks. In addition, interoperability issues with different RRC protocol versions limits opportunities for mobile operators to implement protocol upgrades.
In Evolved Universal Terrestrial Radio Access Network (E-UTRAN), RRM is more distributed than that of UTRAN by implementing RRM functions at the evolved Node B (eNode B) level. As a result, there is increased likelihood that due to protocol mismatches, new radio configurations will not be used in the target eNode B due to lack of support from a source eNode B. The current working assumption for the handover signaling for LTE is to have the same 3-way handshake (e.g., Measurement Report, HO command and HO Complete) as in UMTS, with the above identified difficulties anticipated. In addition to solving these problems, further improvements are desired in connection with inter-eNode B (eNB) handover procedure to allow mobile operators to benefit from frequent protocol upgrades, including physical layer upgrades, allow mobile operators to aggressively employ multi-vendor networks, and enable new radio configurations usage in the target eNode B despite lack of protocol support from the source eNode B.